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1.
The postglacial eruption rate for the Mount Adams volcanic field is ∼0.1 km3/k.y., four to seven times smaller than the average rate for the past 520 k.y. Ten vents have been active since the last main
deglaciation ∼15 ka. Seven high flank vents (at 2100–2600 m) and the central summit vent of the 3742-m stratocone produced
varied andesites, and two peripheral vents (at 2100 and 1200 m) produced mildly alkalic basalt. Eruptive ages of most of these
units are bracketed with respect to regional tephra layers from Mount Mazama and Mount St. Helens. The basaltic lavas and
scoria cones north and south of Mount Adams and a 13-km-long andesitic lava flow on its east flank are of early postglacial
age. The three most extensive andesitic lava-flow complexes were emplaced in the mid-Holocene (7–4 ka). Ages of three smaller
Holocene andesite units are less well constrained. A phreatomagmatic ejecta cone and associated andesite lavas that together
cap the summit may be of latest Pleistocene age, but a thin layer of mid-Holocene tephra appears to have erupted there as
well. An alpine-meadow section on the southeast flank contains 24 locally derived Holocene andesitic ash layers intercalated
with several silicic tephras from Mazama and St. Helens. Microprobe analyses of phenocrysts from the ash layers and postglacial
lavas suggest a few correlations and refine some age constraints. Approximately 6 ka, a 0.07-km3 debris avalanche from the southwest face of Mount Adams generated a clay-rich debris flow that devastated >30 km2 south of the volcano. A gravitationally metastable 2-to 3-km3 reservoir of hydrothermally altered fragmental andesite remains on the ice-capped summit and, towering 3 km above the surrounding
lowlands, represents a greater hazard than an eruptive recurrence in the style of the last 15 k.y.
Received: 24 June 1996 / Accepted: 6 December 1996 相似文献
2.
Cooling and crystallization of lava in open channels, and the transition of Pāhoehoe Lava to 'A'ā 总被引:1,自引:1,他引:0
Samples collected from a lava channel active at Kīlauea Volcano during May 1997 are used to constrain rates of lava cooling
and crystallization during early stages of flow. Lava erupted at near-liquidus temperatures (∼1150 °C) cooled and crystallized
rapidly in upper parts of the channel. Glass geothermometry indicates cooling by 12–14 °C over the first 2 km of transport.
At flow velocities of 1–2 m/s, this translates to cooling rates of 22–50 °C/h. Cooling rates this high can be explained by
radiative cooling of a well-stirred flow, consistent with observations of non-steady flow in proximal regions of the channel.
Crystallization of plagioclase and pyroxene microlites occurred in response to cooling, with crystallization rates of 20–50%
per hour. Crystallization proceeded primarily by nucleation of new crystals, and nucleation rates of ∼104/cm3s are similar to those measured in the 1984 open channel flow from Mauna Loa Volcano. There is no evidence for the large nucleation
delays commonly assumed for plagioclase crystallization in basaltic melts, possibly a reflection of enhanced nucleation due
to stirring of the flow. The transition of the flow surface morphology from pāhoehoe to 'a'ā occurred at a distance of 1.9 km
from the vent. At this point, the flow was thermally stratified, with an interior temperature of ∼1137 °C and crystallinity
of ∼15%, and a flow surface temperature of ∼1100 °C and crystallinity of ∼45%. 'A'ā formation initiated along channel margins,
where crust was continuously disrupted, and involved tearing and clotting of the flow surface. Both observations suggest that
the transition involved crossing of a rheological threshold. We suggest this threshold to be the development of a lava yield
strength sufficient to prevent viscous flow of lava at the channel margin. We use this concept to propose that 'a'ā formation
in open channels requires both sufficiently high strain rates for continued disruption of surface crusts and sufficient groundmass
crystallinity to generate a yield strength equivalent to the imposed stress. In Hawai'i, where lava is typically microlite
poor on eruption, these combined requirements help to explain two common observations on 'a'ā formation: (a) 'a'ā flow fields
are generated when effusion rates are high (thus promoting crustal disruption); and (b) under most eruption conditions, lava
issues from the vent as pāhoehoe and changes to 'a'ā only after flowing some distance, thus permitting sufficient crystallization.
Received: 3 September 1998 / Accepted: 12 April 1999 相似文献
3.
David A. Clague Jonathan T. Hagstrum Duane E. Champion Melvin H. Beeson 《Bulletin of Volcanology》1999,61(6):363-381
The tube-fed pāhoehoe lava flows covering much of the northeast flank of Kīlauea Volcano are named the 'Ailā'au flows. Their
eruption age, based on published and six new radiocarbon dates, is approximately AD 1445. The flows have distinctive paleomagnetic
directions with steep inclinations (40°–50°) and easterly declinations (0°–10°E). The lava was transported ∼40 km from the
vent to the coast in long, large-diameter lava tubes; the longest tube (Kazumura Cave) reaches from near the summit to within
several kilometers of the coast near Kaloli Point. The estimated volume of the 'Ailā'au flow field is 5.2±0.8 km3, and the eruption that formed it probably lasted for approximately 50 years. Summit overflows from Kīlauea may have been
nearly continuous between approximately AD 1290 and 1470, during which time a series of shields formed at and around the summit.
The 'Ailā'au shield was either the youngest or the next to youngest in this series of shields. Site-mean paleomagnetic directions
for lava flows underlying the 'Ailā'au flows form only six groups. These older pāhoehoe flows range in age from 2750 to <18,000
BP, and the region was inundated by lava flows only three times in the past 5000 years. The known intervals between eruptive
events average ∼1600 years and range from ∼1250 years to >2200 years. Lava flows from most of these summit eruptions also
reached the coast, but none appears as extensive as the 'Ailā'au flow field. The chemistry of the melts erupted during each
of these summit overflow events is remarkably similar, averaging approximately 6.3 wt.% MgO near the coast and 6.8 wt.% MgO
near the summit. The present-day caldera probably formed more recently than the eruption that formed the 'Ailā'au flows (estimated
termination ca. AD 1470). The earliest explosive eruptions that formed the Keanakāko'i Ash, which is stratigraphically above
the 'Ailā'au flows, cannot be older than this age.
Received: 10 October 1998 / Accepted: 12 May 1999 相似文献
4.
Michael O. Garcia Ken H. Rubin Marc D. Norman J. Michael Rhodes David W. Graham David W. Muenow Khalil Spencer 《Bulletin of Volcanology》1998,59(8):577-592
Samples of basalt were collected during the Rapid Response cruise to Loihi seamount from a breccia that was probably created
by the July to August 1996 Loihi earthquake swarm, the largest swarm ever recorded from a Hawaiian volcano. 210Po–210Pb dating of two fresh lava blocks from this breccia indicates that they were erupted during the first half of 1996, making
this the first documented historical eruption of Loihi. Sonobuoys deployed during the August 1996 cruise recorded popping
noises north of the breccia site, indicating that the eruption may have been continuing during the swarm. All of the breccia
lava fragments are tholeiitic, like the vast majority of Loihi's most recent lavas. Reverse zoning at the rim of clinopyroxene
phenocrysts, and the presence of two chemically distinct olivine phenocryst populations, indicate that the magma for the lavas
was mixed just prior to eruption. The trace element geochemistry of these lavas indicates there has been a reversal in Loihi's
temporal geochemical trend. Although the new Loihi lavas are similar isotopically and geochemically to recent Kilauea lavas
and the mantle conduits for these two volcanoes appear to converge at depth, distinct trace element ratios for their recent
lavas preclude common parental magmas for these two active volcanoes. The mineralogy of Loihi's recent tholeiitic lavas signify
that they crystallized at moderate depths (∼8–9 km) within the volcano, which is approximately 1 km below the hypocenters
for earthquakes from the 1996 swarm. Taken together, the petrological and seismic evidence indicates that Loihi's current
magma chamber is considerably deeper than the shallow magma chamber (∼3–4 km) in the adjoining active shield volcanoes.
Received: 21 August 1997 / Accepted: 15 February 1998 相似文献
5.
Mary E. MacKay Scott K. Rowland Peter J. Mouginis-Mark Harold Garbeil 《Bulletin of Volcanology》1998,60(4):239-251
We use a digital elevation model (DEM) derived from interferometrically processed SIR-C radar data to estimate the thickness
of massive trachyte lava flows on the east flank of Karisimbi Volcano, Rwanda. The flows are as long as 12 km and average
40–60 m (up to >140 m) in thickness. By calculating and subtracting a reference surface from the DEM, we derived a map of
flow thickness, which we used to calculate the volume (up to 1 km3 for an individual flow, and 1.8 km3 for all the identified flows) and yield strength of several flows (23–124 kPa). Using the DEM we estimated apparent viscosity
based on the spacing of large folds (1.2×1012 to 5.5×1012 Pa s for surface viscosity, and 7.5×1010 to 5.2×1011 Pa s for interior viscosity, for a strain interval of 24 h). We use shaded-relief images of the DEM to map basic flow structures
such as channels, shear zones, and surface folds, as well as flow boundaries. The flow thickness map also proves invaluable
in mapping flows where flow boundaries are indistinct and poorly expressed in the radar backscatter and shaded-relief images.
Received: 6 September 1997 / Accepted: 15 May 1998 相似文献
6.
Michael O. Garcia J. M. Rhodes Frank A. Trusdell Aaron J. Pietruszka 《Bulletin of Volcanology》1996,58(5):359-379
The Puu Oo eruption has been remarkable in the historical record of Kilauea Volcano for its duration (over 13 years), volume
(>1 km3) and compositional variation (5.7–10 wt.% MgO). During the summer of 1986, the main vent for lava production moved 3 km down
the east rift zone and the eruption style changed from episodic geyser-like fountaining at Puu Oo to virtually continuous,
relatively quiescent effusion at the Kupaianaha vent. This paper examines this next chapter in the Puu Oo eruption, episodes
48 and 49, and presents new ICP-MS trace element and Pb-, Sr-, and Nd-isotope data for the entire eruption (1983–1994). Nearly
aphyric to weakly olivine-phyric lavas were erupted during episodes 48 and 49. The variation in MgO content of Kupaianaha
lavas erupted before 1990 correlates with changes in tilt at the summit of Kilauea, both of which probably were controlled
by variations in Kilauea's magma supply rate. These lavas contain euhedral olivines which generally are in equilibrium with
whole-rock compositions, although some of the more mafic lavas which erupted during 1990, a period of frequent pauses in the
eruption, accumulated 2–4 vol.% olivine. The highest forsterite content of olivines (∼85%) in Kupaianaha lavas indicates that
the parental magmas for these lavas had MgO contents of ∼10 wt.%, which equals the highest observed value for lavas during
this eruption. The composition of the Puu Oo lavas has progressively changed during the eruption. Since early 1985 (episode
30), when mixing between an evolved rift zone magma and a more mafic summit reservoir-derived magma ended, the normalized
(to 10 wt.% MgO) abundances of highly incompatible elements and CaO have systematically decreased with time, whereas ratios
of these trace elements and Pb, Sr, and Nd isotopes, and the abundances of Y and Yb, have remained relatively unchanged. These
results indicate that the Hawaiian plume source for Puu Oo magmas must be relatively homogeneous on a scale of 10–20 km3 (assuming 5–10% partial melting), and that localized melting within the plume has apparently progressively depleted its incompatible
elements and clinopyroxene component as the eruption continued. The rate of variation of highly incompatible elements in Puu
Oo lavas is much greater than that observed for Kilauea historical summit lavas (e.g., Ba/Y 0.09 a–1 vs ∼0.03 a–1). This rapid change indicates that Puu Oo magmas did not mix thoroughly with magma in the summit reservoir. Thus, except
for variable amounts of olivine fractionation, the geochemical variation in these lavas is predominantly controlled by mantle
processes.
Received: 8 March 1996 / Accepted: 30 April 1996 相似文献
7.
We present thermal measurements made by high spatial resolution ground-based (a hand-held thermal camera) and low spatial
resolution space-based (MODIS) instruments for a lava flow field active during the last phase of the May–July 2003 eruption
at Piton de la Fournaise (La Réunion). Multiple oblique ground-based thermal images were merged to provide full coverage of
the flow-field. These were then corrected for path length attenuation and orthorectified, allowing the at-surface radiance
emitted by the flow-field to be estimated. Comparison with the radiance recorded by the MODIS sensors during the eruption
reveals that, for clear-sky conditions and moderate-to-low viewing angles (satellite zenith <40°), the satellite measurements
represent ∼90% of the at-surface radiance, and thus represent valuable data for quantifying volcanic thermal anomalies. Nevertheless,
extreme viewing geometries and the presence of clouds strongly affect the radiance reaching the sensor and affected data from
94% of the overpasses. Ground-based thermal data were used to investigate an empirical relationship between the radiant heat
flux and lava discharge rate during the emplacement of pahoehoe flows. While the average radiation temperature for flow surface
that were 6–24 h old ranged between 500 K and 625 K, the ratio between radiative heat flux and Time-Averaged lava Discharge
Rate (TADR) ranged between 1.5 × 108 J m−3 and 3.5 × 108 J m−3. This relationship was used to estimate TADR values from optimal MODIS data and produced results in line with those obtained
from GPS surveys (Coppola et al. 2005). Our results underscore the importance of ground-based thermal analysis for the interpretation of satellite measurements,
particularly in terms of calculating discharge rate trends. 相似文献
8.
Shiveluch Volcano, located in the Central Kamchatka Depression, has experienced multiple flank failures during its lifetime,
most recently in 1964. The overlapping deposits of at least 13 large Holocene debris avalanches cover an area of approximately
200 km2 of the southern sector of the volcano. Deposits of two debris avalanches associated with flank extrusive domes are, in addition,
located on its western slope. The maximum travel distance of individual Holocene avalanches exceeds 20 km, and their volumes
reach ∼3 km3. The deposits of most avalanches typically have a hummocky surface, are poorly sorted and graded, and contain angular heterogeneous
rock fragments of various sizes surrounded by coarse to fine matrix. The deposits differ in color, indicating different sources
on the edifice. Tephrochronological and radiocarbon dating of the avalanches shows that the first large Holocene avalanches
were emplaced approximately 4530–4350 BC. From ∼2490 BC at least 13 avalanches occurred after intervals of 30–900 years. Six
large avalanches were emplaced between 120 and 970 AD, with recurrence intervals of 30–340 years. All the debris avalanches
were followed by eruptions that produced various types of pyroclastic deposits. Features of some surge deposits suggest that
they might have originated as a result of directed blasts triggered by rockslides. Most avalanche deposits are composed of
fresh andesitic rocks of extrusive domes, so the avalanches might have resulted from the high magma supply rate and the repetitive
formation of the domes. No trace of the 1854 summit failure mentioned in historical records has been found beyond 8 km from
the crater; perhaps witnesses exaggerated or misinterpreted the events.
Received: 18 August 1997 / Accepted: 19 December 1997 相似文献
9.
Eruptive style of the young high-Mg basaltic-andesite Pelagatos scoria cone,southeast of México City 总被引:1,自引:1,他引:0
The eruption of the Pelagatos scoria cone in the Sierra Chichinautzin monogenetic field near the southern suburbs of Mexico
City occurred less than 14,000 years ago. The eruption initiated at a fissure with an effusive phase that formed a 7-km-long
lava flow, and continued with a phase of alternating and/or simultaneous explosive and effusive activity that built a 50-m-high
scoria cone on the western end of the fissure and formed a compound lava flow-field near the vent. The eruption ended with
the emplacement of a short lava flow that breached the cone and was accompanied by weak explosions at the crater. Products
consist of a microlite-rich high-Mg basaltic andesite. Samples were analyzed to determine the magma’s initial properties as
well as the effects of degassing-induced crystallization on eruptive style. Although distal ash fallout deposits from this
eruption are not preserved, a recent quarry exposes a large section of the scoria cone. Detailed study of exposed layers allows
us to elucidate the mode of cone-building activity. Petrological and textural data, combined with models calibrated by experimental
work and melt-inclusion analyses of similar magmas elsewhere, indicate that the magma was initially hot (>1,200°C), gas-rich
(up to 5 wt.% H2O), crystal-poor (~10 vol.% Fo90 olivine phenocrysts) and thus poorly viscous (40–80 Pa s). During the early phase, low magma ascent velocity at the fissure
vent allowed low-viscosity magma to degas and crystallize during ascent, producing lava flows with elevated crystal contents
at T < 1,100°C, and blocky surfaces. Later, the closure of the fissure by cooling dikes focused the magma flow at a narrow section
of the fissure. This led to an increased magma ascent velocity. Rapid and shallow degassing (<3 km deep) triggered ~40 vol.%
microlite crystallization. Limited times for gas-escape and higher magma viscosity (6 × 105–4 × 106 Pa s) drove strong explosions of highly (60–80 vol.%) and finely vesicular magma. Coarse clasts broke on landing, which implies
brittle behavior due to complete solidification. This requires sufficient time to cool and in turn implies ejection heights
of over 1 km, which is much higher than “normal” Strombolian activity. Hence, magma viscosity significantly impacts eruption style at monogenetic volcanoes because it affects the kinetics
of shallow degassing. The long-lasting eruptions of Jorullo and Paricutin, which produced similar magmas in western México,
were more explosive. This can be related to higher magma fluxes and total erupted volumes. Implications of this study are
important because basaltic andesites are commonly erupted to form monogenetic scoria cones of the Trans-Mexican Volcanic Belt. 相似文献
10.
Gianluca Sottili Danilo M. Palladino Mario Gaeta Matteo Masotta 《Bulletin of Volcanology》2012,74(1):163-186
Maar volcanoes represent a common volcano type which is produced by the explosive interaction of magma with external water.
Here, we provide information on a number of maars in the ultrapotassic Sabatini Volcanic District (SVD, Roman Province) as
young as ∼90 ka. The SVD maars are characterised in terms of crater and ejecta ring morphologies, eruptive successions and
magma compositions, in light of the local substrate settings, with the aim of assessing magma–water interaction conditions,
eruption energetics and genetic mechanisms. Feeder magmas spanned the whole SVD differentiation trend from trachybasalts–shoshonites
to phonolites. From the ejected lithic fragments from aquifer rocks, the range of depth of magma–water explosive interaction
is estimated to have been mostly at ∼400–600 m below ground level, with a single occurrence of surficial interaction in palustrine–lacustrine
environment. In particular, the interaction with external water may have triggered the explosive behaviour of poorly differentiated
magmas, whereas it may have acted only as a late controlling factor of the degree of fragmentation and eruption style for
the most differentiated magma batches during low-flux ascent in an incipiently fragmented state. Crater sizes, ejecta volumes
and ballistic data allow a reconstruction of the energy budget of SVD maar-forming eruptions. Erupted tephra volumes from
either monogenetic or polygenetic maars ranged 0.004–0.07 km3 during individual maar-forming eruptions, with corresponding total magma thermal energies of 8 × 1015–4 × 1017 J. Based on energy partitioning and volume balance of erupted magmas and lithic fractions vs. crater holes, we consider the
different contributions of explosive excavation of the substrate vs. subsidence in forming the SVD maar craters. Following
available models based on crater sizes, highly variable fractions (5–50%) of the magma thermal energies would have been required
for crater excavation. It appears that subsidence may have played a major role in some SVD maars characterised by low lithic
contents, whilst substrate excavation became increasingly significant with increasing degrees of aquifer fragmentation. 相似文献
11.
A new airborne radar technique can generate digital topographic data for volcanoes at a scale of 10 m spatial and 1–5 m vertical, with a swath width of 6.4 km. Called TOPSAR, the intrument is an interferometric radar flown on the NASA DC-8 aircraft. TOPSAR data permit the quantification of volcano slopes, volumes, and heights, and as such will be valuable for the analysis of lava flows, domes, and lahar channels. This instrument will be flown over several volcanoes in the near future, providing volcanologists with valuable data sets for the analysis of high-resolution topography. We briefly illustrate the potential use of TOPSAR data through examples from Mt Somma and Vesuvius, Italy. 相似文献
12.
Vesiculation path of ascending magma in the 1983 and the 2000 eruptions of Miyakejima volcano, Japan 总被引:1,自引:0,他引:1
The vesiculation of magma during the 1983 eruption of Miyakejima Volcano, Japan, is discussed based on systematic investigations
of water content, vesicularity, and bubble size distribution for the products. The eruption is characterized by simultaneous
lava effusion and explosive sub-plinian (‘dry’) eruptions with phreatomagmatic (‘wet’) explosions. The magmas are homogeneous
in composition (basaltic andesite) and in initial water content (H2O = 3.9±0.9 wt%), and residual groundmass water contents for all eruption styles are low (H2O <0.4 wt%) suggestive of extensive dehydration of magma.
For the scoria erupted during simultaneous ‘dry’ and ‘wet’ explosive eruptions, inverse correlation was observed between vesicularity
and residual water content. This relation can be explained by equilibrium exsolution and expansion of ca. 0.3 wt% H2O at shallow level with different times of quenching, and suggests that each scoria with different vesicularity, which was
quenched at a different time, provides a snapshot of the vesiculation process near the point of fragmentation. The bubble
size distribution (BSD) varies systematically with vesicularity, and total bubble number density reaches a maximum value at
vesicularity Φ ∼ 0.5. At Φ ∼ 0.5, a large number of bubbles are connected with each other, and the average thickness of bubble
walls reaches the minimum value below which they would rupture. These facts suggest that vesiculation advanced by nucleation
and growth of bubbles when Φ < 0.5, and then by expansion of large bubbles with coalescence of small ones for Φ > 0.5, when
bubble connection becomes effective.
Low vesicularity and low residual water content of lava and spatter (Φ < 0.1, H2O < 0.1 wt%), and systematic decrease in bubble number density from scoria through spatter to lava with decrease in vesicularity
suggest that effusive eruption is a consequence of complete degassing by bubble coalescence and separation from magma at shallow
levels when magma ascent rate is slow.
相似文献
T. ShimanoEmail: |
13.
Additional data from proximal areas enable a reconstruction of the stratigraphy and the eruptive chronology of phases III
and IV of the 1982 eruption of El Chichón Volcano. Phase III began on 4 April at 0135 GMT with a powerful hydromagmatic explosion
that generated radially fast-moving (∼100 ms–1) pyroclastic clouds that produced a surge deposit (S1). Due to the sudden reduction in the confining pressure the process
continued by tapping of magma from a deeper source, causing a new explosion. The ejected juvenile material mixed with large
amounts of fragmented dome and wall rock, which were dispersed laterally in several pulses as lithic-rich block-and-ash flow
(F1). Partial evacuation of juvenile material from the magmatic system prompted the entrance of external water to generate
a series of hydromagmatic explosions that dispersed moisture-rich surge clouds and small-volume block-and-ash flows (IU) up
to distances of 3 km from the crater. The eruption continued by further decompression of the magmatic system, with the ensuing
emission of smaller amounts of gas-rich magma which, with the strong erosion of the volcanic conduit, formed a lithic-rich
Plinian column that deposited fallout layer B. Associated with the widening of the vent, an increase in the effective density
of the uprising column took place, causing its collapse. Block-and-ash flows arising from the column collapse traveled along
valleys as a dense laminar flow (F2). In some places, flow regime changes due to topographic obstacles promoted transformation
into a turbulent surge (S2) which attained minimum velocities of approximately 77 ms–1 near the volcano. The process continued with the formation of a new column on 4 April at 1135 GMT (phase IV) that emplaced
fall deposit C and was followed by hydromagmatic explosions which produced pyroclastic surges (S3).
Received: 13 May 1996 / Accepted: 12 November 1996 相似文献
14.
Yuri Taran Tobias P. Fischer Boris Pokrovsky Yuji Sano Maria Aurora Armienta Jose Luis Macias 《Bulletin of Volcanology》1998,59(6):436-449
The 1982 eruption of El Chichón volcano ejected more than 1 km3 of anhydrite-bearing trachyandesite pyroclastic material to form a new 1-km-wide and 300-m-deep crater and uncovered the
upper 500 m of an active volcano-hydrothermal system. Instead of the weak boiling-point temperature fumaroles of the former
lava dome, a vigorously boiling crater spring now discharges / 20 kg/s of Cl-rich (∼15 000 mg/kg) and sulphur-poor ( / 200 mg/kg
of SO4), almost neutral (pH up to 6.7) water with an isotopic composition close to that of subduction-type magmatic water (δD=–15‰,
δ18O=+6.5‰). This spring, as well as numerous Cl-free boiling springs discharging a mixture of meteoric water with fumarolic
condensates, feed the crater lake, which, compared with values in 1983, is now much more diluted (∼3000 mg/kg of Cl vs 24 030 mg/kg),
less acidic (pH=2.6 vs 0.56) and contains much lower amounts of S ( / 200 mg/kg of SO4, vs 3550 mg/kg) with δ34S=0.5–4.2‰ (+17‰ in 1983). Agua Caliente thermal waters, on the southeast slope of the volcano, have an outflow rate of approximately
100 kg/s of 71 °C Na–Ca–Cl water and are five times more concentrated than before the eruption (B. R. Molina, unpublished
data). Relative N2, Ar and He gas concentrations suggest extensional tectonics for the El Chichón volcanic centre. The 3He/4He and 4He/20Ne ratios in gases from the crater fumaroles (7.3Ra, 2560) and Agua Caliente hot springs (5.3Ra, 44) indicate a strong magmatic contribution. However, relative concentrations of reactive species are typical of equilibrium
in a two-phase boiling aquifer. Sulphur and C isotopic data indicate highly reducing conditions within the system, probably
associated with the presence of buried vegetation resulting from the 1982 eruption. All Cl-rich waters at El Chichón have
a common source. This water has the appearence of a "partially matured" magmatic fluid: condensed magmatic vapour neutralized
by interaction with fresh volcaniclastic deposits and depleted in S due to anhydrite precipitation. Shallow ground waters
emerging around the volcano from the thick cover of fresh pumice deposits (Red waters) are Ca–SO4–rich and have a negative oxygen isotopic shift, probably due to ongoing formation of clay at low temperatures.
Received: 21 July 1997 / Accepted: 4 December 1997 相似文献
15.
Karen Fontijn Gerald G. J. Ernst Costanza Bonadonna Marlina A. Elburg Evelyne Mbede Patric Jacobs 《Bulletin of Volcanology》2011,73(9):1353-1368
The ~4-ka trachytic Rungwe Pumice (RP) deposit from Rungwe Volcano in South-Western Tanzania is the first Plinian-style deposit
from an African volcano to be closely documented focusing on its physical characterization. The RP is a mostly massive fall
deposit with an inversely graded base. Empirical models suggest a maximum eruption column height H
T of 30.5–35 km with an associated peak mass discharge rate of 2.8–4.8 × 108 kg/s. Analytical calculations result in H
T values of 33 ± 4 km (inversion of TEPHRA2 model on grain size data) corresponding to mass discharge ranging from 2.3 to 6.0 × 108 kg/s. Lake-core data allow extrapolation of the deposit thinning trend far beyond onland exposures. Empirical fitting of
thickness data yields volume estimates between 3.2 and 5.8 km3 (corresponding to an erupted mass of 1.1–2.0 × 1012 kg), whereas analytical derivation yields an erupted mass of 1.1 × 1012 kg (inversion of TEPHRA2 model). Modelling and dispersal maps are consistent with nearly no-wind conditions during the eruption.
The plume corner is estimated to have been ca. 11–12 km from the vent. After an opening phase with gradually increasing intensity,
a high discharge rate was maintained throughout the eruption, without fountain collapse as is evidenced by a lack of pyroclastic
density current deposits. 相似文献
16.
Age spectra from 40Ar/39Ar incremental heating experiments yield ages of 298 ± 25 ka and 310 ± 31 ka for transitional composition lavas from two cones
on submarine Mahukona Volcano, Hawaii. These ages are younger than the inferred end of the tholeiitic shield stage and indicate
that the volcano had entered the postshield alkalic stage before going extinct. Previously reported elevated helium isotopic
ratios of lavas from one of these cones were incorrectly interpreted to indicate eruption during a preshield alkalic stage.
Consequently, high helium isotopic ratios are a poor indicator of eruptive stage, as they occur in preshield, shield, and
postshield stage lavas. Loihi Seamount and Kilauea are the only known Hawaiian volcanoes where the volume of preshield alkalic
stage lavas can be estimated.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
17.
Storage and interaction of compositionally heterogeneous magmas from the 1986 eruption of Augustine Volcano, Alaska 总被引:2,自引:0,他引:2
Diana C. Roman Katharine V. Cashman Cynthia A. Gardner Paul J. Wallace John J. Donovan 《Bulletin of Volcanology》2006,68(3):240-254
Compositional heterogeneity (56–64 wt% SiO2 whole-rock) in samples of tephra and lava from the 1986 eruption of Augustine Volcano, Alaska, raises questions about the
physical nature of magma storage and interaction beneath this young and frequently active volcano. To determine conditions
of magma storage and evolutionary histories of compositionally distinct magmas, we investigate physical and chemical characteristics
of andesitic and dacitic magmas feeding the 1986 eruption. We calculate equilibrium temperatures and oxygen fugacities from
Fe-Ti oxide compositions and find a continuous range in temperature from 877 to 947°C and high oxygen fugacities (ΔNNO=1–2)
for all magmas. Melt inclusions in pyroxene phenocrysts analyzed by Fourier-transform infrared spectroscopy and electron probe
microanalysis are dacitic to rhyolitic and have water contents ranging from <1 to ∼7 wt%. Matrix glass compositions are rhyolitic
and remarkably similar (∼75.9–76.6 wt% SiO2) in all samples. All samples have ∼25% phenocrysts, but lower-silica samples have much higher microlite contents than higher-silica
samples. Continuous ranges in temperature and whole-rock composition, as well as linear trends in Harker diagrams and disequilibrium
mineral textures, indicate that the 1986 magmas are the product of mixing between dacitic magma and a hotter, more mafic magma.
The dacitic endmember is probably residual magma from the previous (1976) eruption of Augustine, and we interpret the mafic
endmember to have been intruded from depth. Mixing appears to have continued as magmas ascended towards the vent. We suggest
that the physical structure of the magma storage system beneath Augustine contributed to the sustained compositional heterogeneity
of this eruption, which is best explained by magma storage and interaction in a vertically extensive system of interconnected
dikes rather than a single coherent magma chamber and/or conduit. The typically short repose period (∼10 years) between Augustine's
recent eruptive pulses may also inhibit homogenization, as short repose periods and chemically heterogeneous magmas are observed
at several volcanoes in the Cook Inlet region of Alaska. 相似文献
18.
We have used a suite of remotely sensed data, numerical lava flow modeling, and field observations to determine quantitative characteristics of the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Galápagos Islands. Flank lava flow areas, volumes, instantaneous effusion rates, and average effusion rates were all determined for these two eruptions, for which only limited syn-eruptive field observations are available. Using data from SPOT, TOPSAR, ERS-1, and ERS-2, we determined that the 1995 Fernandina flow covers a subaerial area of 6.5×106 m2 and has a subaerial dense rock equivalent (DRE) volume of 42×106 m3. Field observations, ATSR satellite data, and the FLOWGO numerical model allow us to determine that the effusion rate declined exponentially from a high of ~60–200 m3 s-1 during the first few hours to <5 m3 s-1 prior to ceasing after 73 days, with a mean effusion rate of 4–16 m3 s-1. Integrating the ATSR-derived, exponentially declining effusion rate over the eruption duration produces a total (subaerial + submarine) DRE volume of between 27 and 100×106 m3, the range in values being due to differing assumptions about heat loss characteristics; only values in the higher part of this range are consistent with the independently derived subaerial volume. Using SPOT, TOPSAR, ERS-1, and ERS-2 data, we determine that the 1998 Cerro Azul flow is 16 km long, covers 16 km2, and has a DRE volume of 54×106 m3. FLOWGO produces at-vent velocity and effusion rate values of 11 m s-1 and ~600 m3 s-1, respectively. The velocity value agrees well with the 12 m s-1 estimated in the field. The mean effusion rate (total DRE volume/duration) was 7–47 m3 s-1. Dike dimensions, fissure lengths, and pressure gradients along the conduit based on magma chamber depth estimates of 3–5 km produce mean effusion rates for the two eruptions that range over nearly four orders of magnitude, the range being due to uncertainty in the magma viscosity, dike dimensions, and pressure gradient between magma chamber and vent. Although somewhat consistent with mean effusion rates from other techniques, their wide range makes them less useful. The exponentially declining effusion rates during both eruptions are consistent with release of elastic strain being the driving mechanism of the eruptions. Our results provide independent input parameters for previously published theoretical relationships between magma chamber pressurization and eruption rates that constrain chamber volumes and increases in volume prior to eruption, as well as time constants of exponential decay during the eruption. The results and theoretical relationships combine to indicate that at both volcanoes probably 25–30% of the volumetric increase in the magma chamber erupted as lava onto the surface. In both eruptions the lava flow volumes are less than 1% of the magma chamber volume. 相似文献
19.
Tephra dispersal and eruption dynamics of wet and dry phases of the 1875 eruption of Askja Volcano, Iceland 总被引:1,自引:1,他引:0
The 1875 rhyolitic eruption of Askja volcano in Iceland was a complex but well-documented silicic explosive eruption. Eyewitness
chronologies, coupled with examination of very proximal exposures and historical records of distal deposit thickness, provide
an unusual opportunity for study of Plinian and phreatoplinian eruption and plume dynamics. The ∼ 17 hour-long main eruption
was characterized by abrupt and reversible shifts in eruption style, e.g., from ‘wet’ to ‘dry’ eruption conditions, and transitions
from fall to flow activity. The main eruption began with a ‘dry’ subplinian phase (B), followed by a shift to a very powerful
phreatoplinian ‘wet’ eruptive phase (C1). A shift from sustained ‘wet’ activity to the formation of ‘wet’ pyroclastic density
currents followed with the C2 pyroclastic density currents, which became dryer with time. Severe ground shaking accompanied
a migration in vent position and the onset of the intense ‘dry’ Plinian phase (D). Each of the fall units can be modeled using
the segmented exponential thinning method (Bonadonna et al. 1998), and three to five segments have been recognized on a semilog plot of thickness vs. area1/2. The availability of very proximal and far-distal thickness data in addition to detailed observations taken during this eruption
has enabled calculations of eruption parameters such as volumes, intensities and eruption column heights. This comprehensive
dataset has been used here to assess the bias of volume calculations when proximal and distal data are missing, and to evaluate
power-law and segmented exponential thinning methods using limited datasets. 相似文献
20.
J. E. Guest P. D. Spudis R. Greeley G. J. Taylor S. M. Baloga 《Bulletin of Volcanology》1995,57(3):179-184
The basaltic Kaupulehu 1800–1801 lava flow of Hualalai Volcano, Hawaii contains abundant ultramafix xenoliths. Many of these xenoliths occur as bedded layers of semi-rounded nodules, each thinly coated with a veneer (typically 1 mm thick) of lava. The nodule beds are analogous to cobble deposits of fluvial sedimentary systems. Although several mechanisms have been proposed for the formation of the nodule beds, it was found that, at more than one locality, the nodule beds are overbank levee deposits. The geological occurrence of the nodules, certain diagnostic aspects of the flow morphology and consideration of the inferred emplacement process indicate that the Kaupulehu flow had an exceptionally low viscosity on eruption and that the flow of the lava stream was extremely rapid, with flow velocities of at least 10 m s-1 (more than 40 km h-1). This flow is the youngest on Hualalai Volcano and future eruptions of a similar type would pose considerable hazard to life as well as property. 相似文献